One of factors to hinder spread of wind power generation is geographical constraints, for example, in Japan.
One of the geographical constraints is that in Japan a wind velocity and a wind direction rapidly changes because of its mountain weather, and thus it is difficult to maintain a stable output of a wind power generation system.
The above factor deteriorates a power generation efficiency per windmill, resulting in an increase in introduction cost of the wind power generation system.
In addition to the above, in Japan, a land of which is small, problems to a locational environment come to appear as wind power generation spreads, and in a case where location near private houses or a community is inevitable, various troubles in relation to noise are apt to occur.
As a measure to stabilize the output or to solve a noise problem as above, there is disclosed a technique of providing a lift improvement device which controls an airflow by having an electrode mounted on a windmill blade to generate plasma, for example.
However, with regard to use of a lift improvement device as above, the lift improvement device is not necessarily required to be actuated in a good wind condition, and in a case where the lift improvement device is provided later, that is, added, to an existing windmill, the windmill being originally designed to be able to generate power at a maximum efficiency without the lift improvement device, the efficiency is not sometimes improved much even if the lift improvement device is added.